Trunked mobile communication systems are known. Such systems typically allocate frequency pairs (transmit and receive) upon receipt of a request for a communication resource from a requesting communication unit. The requesting communication unit then tunes to the granted frequencies and begins transceiving. Such systems are termed frequency division multiple access (FDMA) systems.
Repeaters are provided at the trunked communication system base sites to receive signals from transmitting communication units and to re-transmit the signal. Repeaters are typically provided with relatively tall antenna and are capable of receiving signals from the extremes of the service coverage area. Also in part because of the tall antenna, a signal re-transmitted by the repeater is capable of being received by a target communication unit substantially anywhere within the service coverage area of the trunked communication system.
Once a frequency has been allocated for use, the communication transaction normally occurs through encoding of an audio signal onto a carrier signal consisting of the assigned frequency. Encoding of the audio signal may occur through a variety of methods. Simple encoding techniques include amplitude modulation (AM) and frequency modulation (FM). Trunked systems, in general, operate under FM encoding techniques. AM, on the other hand, is seldom used in trunking systems.
With the advent of digital signal processing (DSP) and associated digital techniques more sophisticated signal encoding methodologies have been developed and used in nontrunked radio systems. One of the developed methodologies is termed spread spectrum. As the name implies, spread spectrum is an encoding technique involving the use of a relatively broad range of frequencies to avoid interference.
Code division multiple access (CDMA) is one of the spread spectrum methodologies. CDMA, on the other hand, is comprised of two separate methodologies, direct sequence spread spectrum (DSSS) and frequency hopping. Frequency hoppers avoid interference by hopping through a range of frequencies, only transmitting on a particular frequency for a short period of time and transmitting redundant or error correction coding with the information signal.
DSSS, on the other hand, utilizes a high rate binary sequence to spread the bandwidth of the desired signal at the transmitter. The same sequence is then used at the receiver to reduce the bandwidth and recover the transmitted signal. The process of bandwidth reduction at the receiver provides suppression of undesired signals or noise thus providing similar rejection of interference as frequency hop spread spectrum signals.
Because spread spectrum systems disperse signals across a range of frequencies, interference is less a problem in such systems. Radio communication systems, in general, suffer from a variety of problems associated with interference from loss of system access to signal blocking. A need exists for a method to apply spread spectrum technology to conventional radio communication systems.